A piezoelectric oscillator which comprises a piezoelectric oscillator sealed in a gas-tight casing and leads connected at one end to the piezoelectric oscillator and having other end portions extending to the outside of the gas-tight casing, is a well known art. The piezoelectric oscillator employed may be a tuning-fork oscillator, a rectangular AT cut oscillator, etc. The gas-tight casing is formed by a substantially cylindrical cap open at one end and a plug having an outer shape capable of being pressure fitted in the inner surface of the open end of the cap, the plug being pressure fitted therein. The cap is formed from Kovar or german silver. The plug comprises a glass body and a metal ring fitted on the periphery of the glass body, and it is penetrated by a plurality of leads of Kovar or like material. The piezoelectric oscillator is coupled by solder of ends of the leads, and is thus supported by the ends thereof. The piezoelectric oscillator is accommodated gas-tightly by pressure fitting the plug in the cap as noted above. The plug thus serves as a gas-tight sealer terminal which seals the piezoelectric oscillator and is connected to external circuitry.
The above piezoelectric oscillator unit is normally manufactured as follows. First, the inner periphery of the open end of the gap, the outer periphery of the plug and the leads are plated with an eutectic solder. Then, paste of a mixture of the eutectic solder and a flux is supplied to portions of the leads that are to extend into the inside of the gas-tight casing, and surface electrodes of the piezoelectric oscillator, formed on the surface thereof, are held in contact with the portions supplied with the paste. The solder paste is then thermally fused to secure the piezoelectric oscillator to the plug. In this way, the piezoelectric oscillator unit is obtained. By plating the outer periphery portion of the plug and the inner periphery portion of the cap with the soft metal solder, the play between these portions can be filled and sealed when the plug is pressure fitted. In addition, by plating the leads with solder, the property of soldering between the surface electrodes of the piezoelectric oscillator and the external circuitry can be improved. By the term "eutectic solder" is meant solder with a tin content of 62 percent by weight, the remainder being lead. In the following description, solders with lead contents of 60 percent by weight or less, inclusive of the eutectic solder, are referred to as low temperature solder. The remainder of the low temperature solder composition other than lead is entirely or mostly tin, and may contain 2 to 3% of an additive such as silver. The melting point of low temperature solder is below 232.degree. C. the melting point of tin.
With the piezoelectric oscillator unit having the structure as described above, the heat resistance is a very important requirement depending on applications. For example, in an application as an element of an SMD (surface-mounted device) on a circuit substrate, reflow of solder is done for the purpose of mounting after sealing. In such a case, it is required that the reflow can be done with possibility of neither gas-tightness failure of the seal portions nor melting of the mounted portion of the piezoelectric oscillator as a result of heating at 240.degree. C. for about 5 seconds (or at 260.degree. C. for 10 seconds or above). The low temperature solder may be used relatively safely for these portions because the cap provides enough mechanical tightening effect for the seal portions to maintain gas tightness so long as the reflow condition is a relatively low temperature or a relatively short period of time. The reflow condition of a relatively low temperature or a relatively short period of time, however, may not effect fusion of the solder in the mounting portion due to the heat capacity thereof. In such a case, the low temperature solder is no longer safe or sufficient heat resistance may not be ensured, depending on the setting of reflow conditions by the user.
In order to improve the heat resistance of the piezoelectric oscillator, the use high lead content solder has been proposed. For example, Japanese Laid-Open Patent Publication No. Sho 64-68007 proposes the use of high temperature solder for both piezoelectric oscillator mount portion and plug/gap seal portions. According to this literature, by the term "high temperature solder" is meant solder with a melting point between 260 and 370.degree. C. and a Sn:Pb ratio of 1:9. Japanese Laid-Open Patent Publication No. Sho 64-62909 proposes forming a lead type layer of a lead alloy only on the outer periphery of a metal ring constituting a plug and, if necessary, forming a silver plating layer on the lead type plating layer for suppressing generation of lead vapor during heating.
Another problem which should be taken into considerations is the shock resistance. Recently, quartz oscillators are finding increasingly extensive applications to portable telephone sets and other portable units to be held with a hand in use, and these applications are producing increasingly stringent market demand for the shock resistance. For example, a unit may need to withstand being dropped several times on a wood plate from a height of 100 to 150 cm.
Several proposals have also been made regarding shock resistance. Japanese Laid-Open Patent Publication No. Sho 61-247775 proposes a technique wherein a quartz oscillator piece is secured to and supported on a plug with a photo-sensitive adhesive with ashore hardness of 90 or below in D scale. Japanese Laid-Open Patent Publication No. Hei 4-78211 proposes mounting additional leads of a material with a Young's modulus of 5.0.times.10 GPa or below to short inner lead sand bonding a quartz oscillator piece to the other ends of the additional leads. In an embodiment of this invention, the additional leads are formed from a solder with a Young's modulus of 1.95.times.10 GPa and have a short plate-like shape. Japanese Patent Publication No. Hei 7-14132 proposes bonding and securing quartz oscillator electrodes and lead conductor supports leading to external circuitry an adhesive, which contains epoxy resin, polybutadiene resin, petroleum type resin and conductive particles and is flexible.
In many cases where the shock resistance requirement is not so stringent, low temperature solder paste is used to support the piezoelectric oscillator. In such cases, by fast heating to secure the piezoelectric oscillator to the plug, very small solder particles called solder balls or solder spatter are spattered and remain inside the casing or attached to the piezoelectric oscillator, giving rise to yield reduction problems such as defective CI (crystal impedance) and defective oscillation at low drive levels.
Specific problems posed in the above described art will now be considered. According to the techniques proposed in Japanese Laid-Open Patent Publication No. Sho 64-68007 and Japanese Laid-Open Patent Publication No. Sho 64-62909, the plug outer periphery metal ring and the lead portions extending to the inside and outside of the gas-tight sealer casing (hereinafter referred to as inner and outer leads, respectively) are plated with high temperature solder by plating in the usual manner. Alloys with high lead contents tend to form a strong oxide film on their surface, making it difficult to obtain soldering when mounting the piezoelectric oscillator on an external circuitry substrate. This is so because the soldering has to be carried out at such a low temperature as to cause no damage to other parts using low temperature solder. For this reason, the outer leads should be stripped off the high temperature solder and dipped in a low temperature solder bath, thus increasing the number of steps and cost of manufacture. Also, the solder layer formed by the dipping lacks uniformity in thickness, possibly making it difficult to bend the outer leads and deteriorating the mounting property due to fluctuations of its amount. The outer leads are bent when surface mounting the piezoelectric oscillator. When surface mounting the piezoelectric oscillator, the peripheral surface of the cap and the ends of the outer leads should be held in close contact with circuitry substrate surface. To attain this, the outer leads should be bent just like a crankshaft such that their height is aligned to the peripheral surface of the cap.
Japanese Laid-Open Patent Publication No. Sho 64-62909, teaches that high lead content alloys generate lead vapor when heated, and that by carrying out reflow when mounting the piezoelectric oscillator, lead is attached thereto achieve a frequency shift. In this connection, the literature teaches that it is inevitable to adopt a partial plating process taking a large number of steps in order to prevent the high temperature solder from attaching to other portions of the plug outer periphery and the cap inner periphery than the pressure fitting portions. However, we have a different opinion in this respect, that is, lead generates only a negligible amount of vapor because the partial vapor pressure of lead is very low at, for instance, 150.degree. C. Although the vapor pressure is slightly higher at 260.degree. C., it poses substantially no problem, because the heating is completed in a short period of time. Thus, substantially no problem arises unless the low temperature solder inside the casing is fused (particularly in large surface area portions) after sealing.
Japanese Laid-Open Patent Publication No. Sho 64-62909 also teaches that silver plating the surface layer improves the soldering property. However, since silver is expensive, it is usually layered to only a very small thickness (i.e., flush plating or the order of 0.1 to 0.2 .mu.m). Therefore, in a bent lead portion the silver layer is broken to expose the underlying high temperature solder and deteriorate the soldering property. Once the piezoelectric oscillator is mounted, silver is lost in the substrate solder, thus disabling the remounting, that is, making it impossible to reuse parts recovered from external circuitry substrates no longer used.
Regarding the shock resistance improvement, the adhesives used in the techniques proposed in Japanese Laid-Open Patent Publication No. Sho 61-247775 and Japanese Patent Publication No. Hei 7-14132, have problems related to control, hardening, step and reliability. For example, although solder mounting can be made such that fused solder is spread to accurately follow the piezoelectric oscillator surface electrode pattern shape, the spreading of non-hardened, i.e., liquid-phase, adhesive can not be regulated, thus giving rise to fluctuations of the bonding position and causing fluctuations of the CI and the bonding strength. According to the techniques proposed in Japanese Laid-Open Patent Publication No. Hei 4-76211, the mounting of the piezoelectric oscillator by the agency of the small-size additional leads, increased the length of the leads and the total length of the piezoelectric oscillator. Besides, the bonding two different kinds of leads is very inferior in operation control property, and not suited for mass production.